Retroviruses can infect a variety of species, including humans. Retroviral DNA integrates into the genome, resulting in persistent infections. Mammalian genomes are also under constant threat by endogenous retroviruses and retroelements. They thus have developed multiple mechanisms to restrict retroviral infections prior to proviral integration. Among the host restriction factors are APOBEC3 proteins with cytidine deaminase activity (CDA) that act during retroviral replication and that inhibit retrotransposition. There are 7 human A3 (hA3) family members (A3A-A3H), while rodents have a single A3 gene. All A3 proteins have CDA domains and can convert cytidines to uridines in reverse-transcribed single-stranded DNA, resulting in mutation of the viral DNA. While the human A3 proteins have been extensively studied in tissue culture, little is known about their action in vivo. Here, I propose to use transgenic mice that I recently created and express 2 human A3 proteins, A3A and A3G, with the goal of determining their ability to restrict retroviruses in vivo. A3A has a potent role in restricting viruses and retroelement retrotransposition in vitro, while A3G inhibits retroviruses such as HIV-1. Moreover, it has been suggested that A3 proteins play a critical role in preventing zoonoses and indeed, several human A3 proteins have been shown to restrict mouse retroviruses in vitro. Because of their potent antiviral activity, increasing A3 activity is currently being investigated as a target of ani-viral therapy but whether this has the potential to lead to cellular DNA damage or to drug-resistant retroviruses is not known. We will thus use these transgenic mice to test their effects on mouse mammary tumor virus (MMTV) and murine leukemia virus (MLV) infection, retroviruses for which the mouse is the natural host and which are only partially restricted by mouse A3 (mA3). Our laboratory (Dr. Susan Ross' laboratory at the University of Pennsylvania) pioneered the use of genetically modified mice, including mA3 knockout mice, to study host-retrovirus interactions in vivo. Thus, this project will provide me with training in retroviruses ad the generation of transgenic mouse models, while drawing on my previous training in viral pathogenesis in mice. This study will provide insight into the function of the A3 proteins in vivo, and also has the potential to create new models for testing therapeutic strategies for treating retroviral infections in humans.